Here is the standard procedure for flashing NVIDIA cards in PC if you have a suitable Mac ROM for your card.

You will need a PCI card in the PC as well with your monitor connected to it so you can see what you are doing.
Using a PCI card as the primary adapter prevents the PC from loading the ROM of the secondary card before flashing.
This can help avoid misflashing of the card due to settings in its PC ROM.
It is possible to flash NVIDIA cards with a Mac ROM without using a PCI graphics card as a primary display adapter, however, depending on mother-board and hardware setup, flashing without a PCI graphics card as a primary display adapter might coorupt the flashing process with a Mac ROM, when preserving some "adapter settings" which are not compatible with the Mac ROM.

• Download the Mac ROM intended for your PC nvidia card from StrangeDogs, here :
WikiDownloads

• Make a windows boot floppy disk. (or any DOS boot media as USB storage or anything your PC can handle to boot)

• Download the latest version of NVFLASH from here,

http://downloads.guru3d.com/download.php?det=850

• Make another floppy disk with nvflash.exe and cwsdpmi.exe on it & also copy the mac rom onto it. Or just copy nvflash.exe and cwsdpmi.exe on your boot media along with the mac ROM. You should rename the mac rom to a max 8 character name like this :
nvmac.rom

• Go into your PC's bios & set your PC to boot from the floppy disk (or other available boot media) & PCI graphics card. Also make sure your PC bios is set to halt on no error.

• Insert the Windows/DOS boot media you made earlier.

• Place the PCI card and the nvidia card you wish to flash in your PC.

• Attach your monitor to the PCI card.

• Boot your PC with the windows startup disk in the floppy drive or use your alternate boot media & your PC should start to DOS with this prompt :
a:\>(might be c:\> if you use an alternate DOS boot media, used as a hard drive)

• Remove the startup disk & replace with the NVFLASH utility & mac ROM disk, if you have it on a separate floppy disk

• At the a prompt type :
a:\>nvflash --check
and note what display adapter number is set for the nvidia card you want to flash, also make sure the ROM chip is supported by the version of nvflash you're using.

if NVFLASH is fine with your card and its ROM chip, then type :
a:\> nvflash --index=X -4 -5 -6 -j nvmac.rom
X is the number set for the Nvidia card you want to flash to Mac, so it is either 0 or 1, according to what nvflash --check displayed.

Don't forget to type a space between nvflash.exe, the switches and the ROM file, it must look exactly as above to work.

• Hit the return key and nvflash should start reprogramming the rom, asking you for confirmation.

• When finished switch off the computer and remove the card.
If you are putting the card in a G4, you need to tape over the pins 3 & 11 on the reverse side of the card to make the card compatible with 4x AGP.
scotch.jpg
scotch.jpg

http://www.fishface.web.aplus.net/images/taping.jpg

Don't do this if the card is going into an AGP G5
Put in your mac, and connect any power connector needed by your card. If you see the grey Apple logo screen you are good to go.

Run some 3D tests and benchmarks to make sure everything is fine.